In central heating and air conditioning systems, a central heating and/or air conditioning unit produces heated or cooled air. This heated or cooled air is directed from the system through a series of ducts throughout a building so as to place the heated or cooled air into the desired location. The heated or cooled air is forced from the heating and/or air conditioning system by means of a fan, blower, or air handler.
Typically, a thermostat is used to actuate the heating unit when the air temperature within the building drops below a desired level, and similarly, the thermostat is used to actuate the air conditioning system when the ambient air temperature within the building rises above a certain level. Actuation of the system causes heated or cooled air to be forced into the building, and the system switched off by the thermostat when the building's air temperature again reaches the desired level.
In many heating and air conditioning systems commonly in use, the thermostat activates the fan and the heating or air conditioning unit simultaneously, and switches off the fan and unit simultaneously, or, particularly with heating systems, the fan may continue to run after the unit is switched off until the plenum reaches a predetermined temperature. The result of this is that all of the heated or cooled air present in the system and in the duct work at the time of shut off is not removed from the duct work by the fan since the fan ceases to operate at the same time as the heating or air conditioning unit ceases to operate, or when the plenum temperature reaches the predetermined level. Accordingly, air which has been heated or cooled by the unit remains in the duct or the unit and is not directed into the building, wasting heated or cooled air.
This problem has been addressed by the prior art. Certain devices cause the fan to continue to be operational for a period of time after the heating or air conditioning unit ceases to operate. This system is not satisfactory in that the blower or fan may operate for longer than is necessary to remove the remaining heated or cooled air from the duct work, or may not operate long enough to remove all of the heated or cooled air from the duct work. Since the optimum time for the fan or blower to continue to operate after the heat or air conditioning system has been deactivated varies according to several factors, including outside ambient air temperatures versus inside ambient air temperatures, a set pre-determined time does not achieve a wholly satisfactory result.
Another approach has been to place a second thermostat within the system or duct work itself. This second thermostat is used to insure that the fan continues to run until a desired temperature is reached. The problem with this type of system is that, if a thermostat alone is used, the fan may be actuated independently of the heat and/or air conditioning system due to ambient air temperature changes. For example, the thermostat which is used to determine the presence of cool air in the duct work, and therefore causes the fan to continue to operate, may be activated by cold, outside air in the winter time. The fan may therefore begin to operate, not in response to removing remaining heated or cooled air from the system, but in response to the outside ambient air temperature affecting the temperature in the duct. A similar situation may result from hot air from the outside in the summer.